Particle beam devices are used for analyzing and examining objects (hereinafter also called samples) in order to obtain insights with regard to the properties and behavior of the objects under specific conditions. One of those particle beam devices is an electron beam device, in particular a scanning electron microscope (also known as SEM).
In an SEM, an electron beam (hereinafter also called primary electron beam) is generated using a beam generator. The electrons of the primary electron beam are accelerated to a predeterminable energy and focused by a beam guiding system, in particular an objective lens, onto a sample to be analyzed (that is to say an object to be analyzed). A high-voltage source having a predeterminable acceleration voltage is used for acceleration purposes. Using a deflection unit, the primary electron beam is guided in a raster-type fashion over a surface of the sample to be analyzed. In this case, the electrons of the primary electron beam interact with the material of the sample to be analyzed. In particular, interaction particles and/or interaction radiation arise(s) as a consequence of the interaction. By way of example, electrons are emitted by the sample to be analyzed (so-called secondary electrons) and electrons of the primary electron beam are backscattered at the sample to be analyzed (so-called backscattered electrons). The secondary electrons and backscattered electrons are detected and used for image generation. An image of the sample to be analyzed is thus obtained.
The interaction radiation comprises X-rays and/or cathodoluminescence light and may be detected with a radiation detector. When measuring X-rays with the radiation detector, in particular energy-dispersive X-ray spectroscopy (also known as EDS or EDX) may be carried out. EDX is an analytical analysis method used for the elemental analysis or chemical characterization.
An ion beam device is also known from the prior art. The ion beam device comprises an ion beam column having an ion beam generator. Ions are generated which are used for processing a sample (for example for removing a layer of the sample or for depositing material on the sample, wherein the material is provided by a gas injection system) or else for imaging.
Furthermore, it is known from the prior art to use combination devices for processing and/or for analyzing a sample, wherein both electrons and ions can be guided onto a sample to be processed and/or to be analyzed. By way of example, it is known for an SEM to be additionally equipped with an ion beam column as mentioned above. The SEM serves, in particular, for observing the processing, but also for further analysis of the processed or non-processed sample. Electrons may also be used for depositing material. This is known as electron beam induced deposition (EBID).
Images generated using a particle beam device, for example an SEM, may have a depth of field. FIG. 1 shows a drawing for further explanation of the depth of field of an image generated using the SEM. Reference sign 1000 denotes a primary electron beam which is focused by an objective lens 1001 onto an object 1002 arranged in an object plane. The primary electron beam 1000 is guided through an opening of an aperture unit 1003. The depth of field DOF is the distance area which is imaged as a sharp image. Therefore, features of the object being arranged in this distance area are in sharp focus and are clearly shown in the image generated using the SEM.
The depth of field DOF may be adjusted in the SEM, for example, by adjusting the working distance WD, which is the distance between the objective lens 1001 and the object plane of the object 1002 and/or by adjusting the opening of the aperture unit 1003. A large depth of field DOF may be obtained using a small opening of the aperture unit 1003, whereas a small depth of field DOF may be obtained using a large opening of the aperture unit 1003. Moreover, a large depth of field DOF may be obtained using a large working distance WD, whereas a small depth of field DOF may be obtained using a small working distance WD.
The angle α shown in FIG. 1 is the angle of aperture. By reducing the angle of aperture, the depth of field DOF is increased. A small angle of aperture, however, would increase the diameter of the primary electron beam due to a diffraction effect of the aperture unit and deteriorate the image resolution of the SEM. Therefore, the depth of field DOF and the diameter of the primary electron beam (and thus the resolution of the particle beam device in the generated image) are in trade-off relation to each other. If the DOF is increased, the resolution of the particle beam device is decreased. Accordingly, if the DOF is decreased, the resolution of the particle beam is increased. The angle of aperture may be adjusted using a condenser unit, as explained further below.
As to the prior art, an SEM is known comprising two condenser units, namely a first condenser unit and a second condenser unit. Each of the first condenser unit and the second condenser unit may be a condenser lens. The depth of field of the images generated using this known SEM is adjustable to a first value and to a second value. If the depth of field is adjusted to the first value, the SEM is operated in a first operation mode. If the depth of field, however, is adjusted to the second value of the depth of field, the SEM is operated in a second operation mode. The first operation mode is the so-called high resolution mode which provides for a high resolution of the image, for example 1 nm. However, the first operation mode provides only for a low depth of field (for example 12 μm). The second operation mode provides for a large depth of field (for example 300 μm). However, the resolution of the image is quite low (for example 3 to 4 nm). Since this SEM known from the prior art provides only for two values of depth of field, the depth of field in the image may fail to meet certain user's needs.
A further SEM for digitally processing an image signal to secure the largest focal depth and the best resolution in accordance with the magnification for observation is also known from the prior art. The known SEM comprises a sample holder for holding a sample, an electron beam source, a plurality of convergence lenses for converging the electron beam emitted from the electron beam source, an objective lens for radiating the converged electron beam as a micro spot on a sample, a scanning coil for scanning the electron beam on the sample, a detector for detecting the sample signal generated from the sample irradiated with the electron beam, an analog-to-digital converter (hereinafter also referred to as A/D converter) for converting the analog detection signal of the detector to a digital signal, a storage unit for storing the digital signal converted by the A/D converter as an image signal, and a display unit for displaying an image associated with the image signal stored in the storage unit. The A/D converter is adapted to switch the number of pixels per screen by changing the sampling rate, and the angle of aperture of the electron beam is changed in accordance with the pixel size (visual field area per pixel) determined in accordance with the number of pixels per screen. The angle of aperture of the electron beam is changed by controlling the convergence lenses and is set in such a manner as to realize the best resolution as determined by the pixel size of the image displayed on the display unit and the largest depth of field for the particular resolution.
This known SEM provides for an adjustable depth of field which is calculated based on data stored in the storage unit. However, the depth of field is only realized with the best possible resolution and, therefore, may not be freely adjusted to provide an image of an object as desired by a user.
As regards prior art, reference is made in particular to US 2006/0226362 A1.
In light of the aforesaid, it is desirable to provide a method for generating an image of an object and a particle beam device for carrying out the method which provide a possibility of selecting a depth of field of the image according to the user's needs.